A need for energy has placed an emphasis on obtaining energy from renewable sources. Organic photovoltaic devices may offer a practical path to achieve low-cost, renewable energy by converting light to electricity. One component of an organic photovoltaic device may be a p-type material. The p-type material may be a donor-acceptor polymer that includes a plurality of alternating electron-rich (donor) units and electron-deficient (acceptor) units. A solar cell may be formed by appropriately coupling the donor-acceptor polymer with an n-type material, an anode, and a cathode. For example, the donor-acceptor polymer may be coupled with the n-type material as a bulk heterojunction between the anode and the cathode.
A donor-acceptor polymer may have a high degree of conjugation and may have a high degree of crystallinity to enable charge separation and transport. The donor-acceptor polymer may also have one or more alkyl side chains covalently coupled to a backbone of the donor-acceptor polymer to improve solubility of the donor-acceptor polymer and enable solution processing into large area films. The backbone of the polymer may include one or more fused ring structures to promote planarity of the donor-acceptor polymer.